In the PCT application PCT/IL2006/001131, published as WO2007/036937 for “Directional Light Transmitter and Receiver”, and in the PCT application PCT/IL2009/000010, published as WO/2009/008399 for “Wireless Laser Power”, and in the PCT Application PCT/IL2012/000230 published as WO/2012/172541 for “Partially Distributed Laser Resonator”, all having a common inventor with the present application, there are shown various aspects of wireless power delivery systems based on distributed laser resonators. This term is used in the current disclosure to describe a laser having its cavity mirrors or end reflectors separated in free space, having a gain medium between the cavity mirrors, and without any specific predefined spatial relationship between the cavity mirrors, such that the laser is capable of operating between randomly positioned end reflectors. The end reflectors need to be retroreflectors for this configuration to lase. In the above mentioned applications, one use of such distributed laser resonators is in transmitting optical power from a centrally disposed transmitter, which for practical purposes, incorporates the gain medium, to mobile receivers positioned remotely from the transmitter, with the end mirrors being positioned within the transmitter and receiver. Such distributed laser resonators use, as the end mirrors of the cavity, simple retroreflectors, such as corner cubes, and cats-eyes and arrays thereof. Retroreflectors differ from plane mirror reflectors in that they have a non-infinitesimal field of view (FOV hereinbelow). An electromagnetic wave front incident on a retroreflector within its FOV is reflected back along a direction parallel to but opposite in direction from the wave's source. The reflection takes place even if the angle of incidence of such a wave on the retroreflector has a value different from zero. This is unlike a plane mirror reflector, which reflects back along the incident path only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence.
Prior art distributed resonator lasers are limited by a very strict tradeoff between FOV and gain. This limitation, which is common to all laser gain medium types, comes from physical limitations which can be described as follows, using a laser with a gain medium having amplification α. A single photon injected into the laser gain medium causes a photons to be emitted therefrom. The gain medium must conserve energy, such that the energy it emits cannot exceed the energy it consumes plus any incoming energy. For that reason, every laser gain medium has a saturation effect. The gain available for a small amount of energy well below the saturation level, injected into the gain medium is called “small signal gain”, and the gain available during steady state operation, in which the laser output and losses are exactly matched, is called the “saturated gain”.
Every laser gain medium amplifies two types of incoming light, light circulating inside the laser resonator, and random photons that may be either spontaneous emission from the gain medium itself or random scattered photons coming from the circulated optical power or from other sources. When the light from random photons is amplified so much by the gain medium that it produces intensities similar to the saturation intensity of the gain medium, then the amplification of random light will significantly reduce the gain available for laser gain. In such a case, increasing the pumping energy will have the following effects:
(i) Worsen thermal problems and energy consumption;
(ii) Increase the population inversion, approximately linearly with pump power, though more generally, less then linearly;
(iii) Increase the gain per mm length approximately linearly;
(iv) Increase the amount of energy lost by amplification of random photons, approximately exponentially; and
(v) Reduce the energy available for amplification of resonating photons.
Thus, there must be some limit of gain for a gain medium, beyond which the gain cannot be increased.
While the level of random photons traveling in directions outside the system's FOV can be limited by use of apertures and other means, apertures cannot be used to limit photons traveling inside the FOV but not towards the receiver. Various techniques have been developed to reduce the repetitive bouncing back and forth of spontaneous photons between internal laser components, other than those photons taking part in the main laser mode. Some such methods are suggested in U.S. Pat. No. 5,936,984 to H. E. Meissner et al, for “Laser rods with undoped flanged end-caps for end-pumped laser applications”. However, it is still impossible to block one-way traveling random photons within the FOV of the system, and for that reason there is an inherent tradeoff between the FOV of a gain medium, and the maximal available gain it can produce.
This phenomenon is summarized in an article by G. J. Linford et al, entitled “Very Long Lasers” published in Applied Optics, Volume 13, No 2, Page 379-390 (1974) as well as in most textbooks on lasers, such as in the classic work by A. E. Siegman entitled “Lasers” published by University Science Books, (1986), both of which are hereby incorporated by reference each in their entirety.
High gain is especially important in distributed resonator lasers, in order for the system to be resilient to losses, some of which are specifically inherent in such lasers, both because of the exposure of the resonator to the environment and because distributed resonators are typically long compared to their beam diameter. Such losses include:
(i) Diffraction losses from small aperture optics;
(ii) Dust, fingerprints and other contaminants;
(iii) Misalignment;
(iv) Absorption during passage of the beam through the air;
(v) Scattering from optical components; and
(vi) Reflections from optical components.
In the above referenced article by Linford et al, the field of view was severely limited in order to achieve high gain. Thus there is stated on page 381 of that article in connection with expression (10) thereof, which relates the laser amplifier single pass small signal gain to the FOV, that:
“The active solid angles of these laser amplifiers were of the order of 10−5 sr. the solid angle corresponds to an active angular field of only a few milliradians. The SF-limited optical gains of the high gain Xenon laser amplifiers were measured to range from 30 dB to 35 dB (single pass gain); this agrees well with the 30 dB amplifier gain limit predicted by expression (10)”
Since distributed resonator lasers of the types described in the above referenced PCT applications, are intended for transmitting power to receivers located over a large area opposite the transmitter, the lasers must have a high FOV, and hence suffer from low gain as a result.
There exist techniques for increasing the FOV, such as the inclusion of a telescope, as suggested in the above referenced WO/2012/172541 for “Partially Distributed Laser Resonator”, and elsewhere, but at the cost of decreasing beam diameter and range, since a smaller beam diameter has a shorter Rayleigh length. However it must be noted that such techniques do not solve the problem of losses arising from amplified spontaneous emission resulting from scattering of laser light by dust, air contamination, optical component contamination, and so on, and do not change the nature of the fundamental limitations presented above.
High gain is extremely important in order for a real system to be operational. Many of the losses inherent to distributed laser resonators, such as those arising from such effects as fingerprints because of their open exposure to the environment, or diffraction losses in a very small receiver with small aperture optical elements, such as would be installed on a mobile telephone, can easily reach the order of 50%. It is to be understood that the term “losses” in this context refers to light that is absorbed, scattered, reflected, or otherwise lost from the main lasing modes. Thus, for example, in the case of fingerprints, most photons are not lost in the conventional sense, but are seen as being lost by the laser since the light is scattered outside the main lasing modes.
There is a relation between the saturation intensity and the optical power level of an operating laser. Saturation intensity Is is a property of the laser gain material, and is defined as the input intensity at which the gain of the optical amplifier drops to exactly half of the small signal gain. The saturation intensity Is can be computed as:
      I    s    =            h      ⁢                          ⁢      υ                      σ        ⁡                  (          υ          )                    ·              τ        s            where:h is Planck's constant,τs is the saturation time constant, which depends on the spontaneous emission lifetimes of the various transitions between the energy levels related to the amplification, andv is the frequency in Hz.In an operating laser the intensity of the beam circulating inside the resonator is typically of the same order of magnitude as the saturation intensity, as it tends to grow until saturation kicks in.
It is very advantageous for a distributed laser system to have the circulating power inside the resonator as low as possible, as this would result in:
(i) Better safety, since the cavity is open to the environment, and all risks are directly proportional to the level of circulating power; and
(ii) A lower laser threshold and improved efficiency, since the laser threshold is proportional to saturation intensity.
Therefore, the saturation intensity of the gain medium of “safety limited” or “efficiency limited” distributed laser system needs to be as low as possible. However low saturation intensity also means that a larger beam area is needed to amplify the required power, since intensity is lower, which in turn means higher threshold power. On the other hand, a lower saturation intensity leads to a lower maximal gain, as explained in the above referenced article by Linford et al, and eventually poses a limit on the field of view.
There therefore exists a need for a distributed resonator laser system which has high gain combined with a large field of view, so that the gain is high enough to be able to overcome the inherent losses of distributed resonators, thus overcoming at least some of the disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.